NDTnet - May 1997, Vol.2 No.05

Ultrasonic imaging of internal defects in composites

W. Hillger DLR Braunschweig, Germany *

Presented on the UTonline Application Workshop in May '97

1. Introduction

2. High resolution ultrasonic scanning system

3. Inspections of thin CFRP laminates

Figure 1: Pulse response of a 50 MHz PVDF-foil transducer, scale: 0.1 V/div. and 50 ns/div.
right Figure 2: Frequency spectra of a glassplate reflector,
1: Broadband, 2: with B-scan filter, 3: with C-scan filter, scale: 1MHz/div.; 4 dB/div.

Figure 3: C-scan recorded without C-filter. right Figure 4: C-scan recorded with C-filter

4. Inspections of sandwich components

Table 1: Description with pulse parameters of the sandwich specimen D
Material CFRP-prepreg / Nomex -core
total thickn./ thickness of core 16 mm/15 mm
dimensions 310*100 mm
time of flight through tot. Thickness. 13.7 µs
core velocity 2300 m/s
skin velocity3100 m/s
pulse width of pulser 0.45µs
RF-A-scans with interface-and back wall echo
    0.1 V/div;
Spectrum of back wall echo
Frequency range (-6 dB)0.42-0.71 MHz
Bandwidth (-6 dB)0.71 MHz
Maximum 0.8 MHz

Fig. 6: C-scan of a honeycomb specimen with internal defects Fig. 7: Horizontal B-scan with defects "C", "N" and "D"

Figure 7 is a B-scan from specimen "D1", recorded along a vertical line extended through defects "C", "N" and "D". The defects are indicated by interruptions of the back-wall echo (16 µs on the time scale) and echo amplitudes between interface and back-wall echo: defect "C" between x=10 and x= 30 mm with echo amplitudes between 8 and 12µs, defect "N" at x = 100 mm and between 4 and 8 µs and defect "D" between x=120 mm and x=160 mm indicating a diagonal direction indication.
Figure 6 presents a C-scan in echo technique of specimen "D1" with back-wall echo evaluation. The all defects "B", "C", "D" and "N" are clearly displayed by amplitude decreases of -8 to -15 dB. Optimised pulse parameters were used.

5. Summary

6. References

  1. D.J. Hagenmaier, R. H. Fassbender, Ultrasonic inspection of carbon-Epoxy composites, Materials Evaluation, 43, April 1985, pp. 556-560.
  2. Y. Bar-Cohen, NDE of Fiber-Reinforced Composite Materials- A Review, Materials Evaluation,44, March 1986, pp. 446- 454.
  3. Hillger, W.: US-Inspections of CFRP-Laminates with high resolution. Non-Destructive Testing, Proc. of the 12th Conf. on NDT, Amsterdam, April 23-28, 1989, Vol. 2, S. 1561-1566
    [4] Hillger, W.: High Frequency Ultrasonic Inspection of CFRP-laminates. Ultrasonic International, Madrid, 3-7 Juli 1989, Conf. Proc. S. 47-54
  4. Hillger, W.: Ultrasonic imaging of damages in CFRP-laminates, Acoustical Imaging, Vol. 19, pp. 575-579, Plenum Press, New York and London 1992.
  5. Hillger, W.: Ultrasonic imaging of internal defects in CFPRP-Components, 6th European conference on Non Destructive Testing. Conference proc. part 1, (1994) pp. 449-453
  6. Hillger, W.: Bildgebende Ultraschall-Prüftechnik für Sandwich-Bauteile, Materialprüfung 37 (1995), Heft 11-12, S. 469-472
  7. Hillger, W.: Optimization of Ultrasonic Pulse Parameters for Honeycomb Sandwich Structures, Brite Euram Project No. BE-5781, Document No. DAMTOS-WP-504-1.0/DLR
  8. Hillger, W., Friederichs, B.: Ultrasonic Inspection of the Helicopter EH 101 Tail Unit, Brite Euram Project No. BE-5781, Document No. DAMTOS-WP-503-1.1/DLR


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